1. Field of the Invention
The present invention relates to a semiconductor laser and a method of fabricating the same, and more particularly, to a vertical cavity surface emitting laser (VCSEL) module having a monitoring photodiode (MPD) and a method of fabricating the same.
2. Description of the Related Art
Vertical cavity surface emitting lasers (VCSELs) have a high coupling efficiency due to a low threshold current and a circular beam shape compared to traditional edge-emitting laser diodes. Also, VCSELs can easily be made into two-dimensional arrays and be tested at the wafer level, thus enabling high volume production at low costs comparable to traditional electronic devices. Thus, VCSELs are emerging as promising alternatives to conventional optical devices for use in optical communication networks and optical sensors. Much research is being focused toward the development of low-priced modules using VCSELs.
A VCSEL is typically coupled to a monitoring photodiode (MPD) such as a positive-intrinsic-negative (PIN)-PD for monitoring the output power of the laser and controlling the output power based on the monitored output power. In this case, it is difficult to achieve low-cost coupling between the VCSEL and the MPD. That is, because a VCSEL emits light perpendicular to the surface of a substrate, it is not easy to monolithically integrate a MPD with the VCSEL so as to detect light. For the monolithic integration of a MPD with a VCSEL, the MPD may be attached to the bottom surface of the VCSEL using wafer fusion or a metallic bonding. Another method is to monolithically grow the layers of the MPD and the VCSEL on a substrate and couple the MPD to a side or bottom of the VCSEL using a device fabrication process.
Of the above methods, wafer fusion may degrade device reliability and manufacturing yield due to defects in fusion that may occur during fabrication process, thus increasing the manufacturing costs. Wafer fusion also causes a voltage drop at a fused interface, thus resulting in increased input voltage.
If a MPD is coupled to a side of a VCSEL, the MPD cannot accurately monitor only the output power of the VCSEL because it detects both spontaneous emission and the output power of the VCSEL. When a MPD is coupled to a bottom surface of a VCSEL, the MPD cannot effectively detect the output power of the VCSEL due to the absorption of the laser beam into the substrate.
Various alternatives for coupling a MPD with a VCSEL using TO-can package have been proposed. However, use of a TO-can package requires accurate alignment between the MPD and the VCSEL in order to efficiently detect light emitted from the VCSEL. This necessarily leads to an increase in the price of the VCSEL modules.